My question is, why are starters necessary & what benefits do they provide?

If the medium in a starter is wort, I don't understand what is different about the smaller quantity in a
starter that is preferable over 5 gallons in a carboy? Is this only something to do when you are trying to
increase the amount of yeast before pitching?

TCJOH recommends re-hydrating dried yeast to improve performance, and John Decarlo mentioned "a strain on
yeast cell walls" when pitching directly into the wort. What does re-hydrating in sterile water do that
your wort wont?